1. Field of the Invention
The present invention relates to a radio base station, which performs radio communications with a mobile station, and a method of detecting overlap of a radio communications parameter. In particular, the present invention relates to a radio base station and a method of detecting overlap of the radio communications parameter, both of which detect whether or not a parameter for radio communications such as a spreading code for each radio base station is assigned in an overlap manner to a plurality of radio base stations.
2. Description of the Related Art
In a radio communications system of multiple access, in which multiple communications are simultaneously performed between a radio base station and mobile stations, each radio communications channel used in the communications needs to be identified. For example, in a radio communications system (what is so called the third generation mobile communications system) based on a code division multiple access (hereinafter, referred to as CDMA), each communications channel is identified with a spreading code (radio communications parameter), which is a random code sequence. Specifically, a channelization code and a scrambling code (hereinafter, referred to as ScC) are set as the spreading code. The channelization code is used for spreading user data in wideband, and the scrambling code is used for preventing interference between cells.
In a wideband code division multiple access (W-CDMA), which is one of the third generation mobile communications systems, 512 scrambling codes (downlink) are specified. Since the number of scrambling codes is limited, the same scrambling code is repeatedly assigned in a system while attempting not to assign the same scrambling code to near cells.
Conventionally, a measure to prevent the same scrambling codes from being assigned to radio base stations was taken based on a simulation using a computer. Specifically, the same scrambling code is not assigned to base stations having overlapping coverage areas which are estimated by the simulation. Here, the coverage area means an area where radio waves can reach from the base station. Accordingly, when a radio base station is to be newly provided in due to an increase in traffic, it is necessary to reconsider the assignment of scrambling codes. Moreover, the assignment of scrambling codes needs to be also reconsidered when a difference occurred between the coverage area based on a simulation using a computer and the actual coverage area.
In the meantime, a method in which a radio communications system (radio base station and mobile stations) autonomously determines scrambling codes of a downlink (for example, in Japanese Patent Application Laid-open Publication No. 2005-175611 (on pages 9 to 10 of FIG. 6)). Specifically, the radio base stations autonomously determine scrambling codes of a downlink. Each mobile station detects the overlap of a scrambling code (downlink) used by a radio base station and a different radio base station located near the radio base station. The mobile station notifies the radio base station of the overlap of the scrambling codes which have been autonomously assigned by the base station. Upon receipt of the notification from the mobile station, the radio base station changes the scrambling code to a different scrambling code.